1. Field of the Invention
The present invention relates to a flat lighting device which can enhance the uniformity of an in-plane brightness level and a liquid crystal display device which uses such a flat lighting device.
2. Background Art
In a transmissive or transflective liquid crystal display device, a flat lighting device is formed on a back surface of the liquid crystal display device in general. This type of flat lighting device is also referred to as a backlight. In the explanation made hereinafter, the explanation is made by describing the flat lighting device also as the backlight. Particularly, with respect to the liquid crystal display device which is mounted on a gaming machine, a portable digital assistant or a mobile phone, a side-edge type backlight which assembles one or a plurality of light emitting diodes (hereinafter also abbreviated as LED) to a light guide plate as a light source has been popularly used. Also an attempt has been made to illuminate an illumination surface (back surface) of the liquid crystal display panel which is arranged above the light guide plate with the brightness distribution having small irregularities by forming a beam control element on a surface (light radiation surface) of the light guide plate.
As a conventional example of the backlight which assembles the LED to the light guide plate, the following backlight is known. JP-A-2004-200072 discloses a beam control element in which concave shapes or convex shapes (crest/grove shapes) having a triangular cross section are formed linearly. JP-A-2004-6187 discloses beam control elements in which concave shapes or convex shapes having an arcuate cross section are formed linearly. JP-A-2004-227956 discloses a beam control element in which concave shapes or convex shapes having a polygonal cross section are linearly formed. JP-A-2003-149639 discloses a beam control element in which embossed shapes having a semicircular, a semi-spherical or triangular-pyramidal cross section are linearly formed. JP-A-2004-199958 discloses dotted beam control elements which are formed by dot printing.
Further, there has been also proposed a backlight in which beam control elements in which concave shapes or convex shapes having a triangular, arcuate or polygonal cross section are formed on the whole surface of the light guide plate concentrically or in an arcuate shape.